The present invention relates to a transfer device for use in an electrophotographic image forming apparatus, and in particular to a transfer device for firstly transferring a toner image as formed on an image carrier to an endless intermediate transfer belt and secondly transferring the toner image from the intermediate transfer belt to a record medium such as a sheet of paper (hereinafter referred to merely as a sheet).
A full-color image forming apparatus forms an image, using a toner of color corresponding to each of a plurality of color image data obtained by color separation from an original color image. More specifically, the image forming apparatus reads the original full-color image through different color filters for the three additive primary colors—red, green, and blue—and produces color image data for the three subtractive primary colors—cyan, magenta, and yellow—and black, respectively. Based on each of the color image data, a developed image is created with a toner of corresponding color. Resulting developed images for the respective colors are accumulated to form a full-color image.
Japanese Patent Application Laid-Open No. H10-039651 discloses a tandem-type full-color image forming apparatus having a semiconductive endless belt and a plurality of (e.g. four) image forming sections. The endless belt is installed rotatably, and the image forming sections each provided for forming a developed image of corresponding color are aligned along an outer circumference of the endless belt. This arrangement allows a full-color image to be formed in at least one full rotation of the endless belt.
There is also known a tandem-type full-color image forming apparatus using an intermediate transfer method. In the image forming apparatus, developed images for the respective colors formed on photoreceptor drums as image carriers in respective image forming sections are accumulated on an outer circumferential surface of an endless belt (an intermediate transfer belt) and then transferred to a sheet, to form a full-color image.
More specifically, toner images are formed on the image carriers (photoreceptor drums) in the respective image forming sections, based on image data for the respective colors obtained by color separation. The toner images are firstly transferred from the photoreceptor drums to the intermediate transfer belt to be accumulated, or first transfer processes are performed. Then, the accumulation of toner images is secondly transferred from the intermediate transfer belt to the sheet, or a second transfer process is performed.
Accordingly, the formation of a full-color image involves the first transfer processes performed in a plurality of, for example four, first transfer regions, and the second transfer process performed in a second transfer region other than the first transfer regions. While following a loop path, the intermediate transfer belt passes through the first transfer regions and the second transfer region, in the order.
Conventionally, each of the first transfer regions has a transfer nip area formed as follows. A first transfer roller, which is flexible, is pressed against a circumferential surface of the photoreceptor drum, which is a rigid body, through the intermediate transfer belt. Part of a circumferential surface of the first transfer roller is thus deformed elastically along the circumferential surface of the photoreceptor drum, so that the intermediate transfer belt is brought into contact with the circumferential surface of the photoreceptor drum over a predetermined contact width to form a transfer nip area. The transfer nip area is provided for transferring a toner image in a stable manner from the outer circumferential surface of the photoreceptor drum to the intermediate transfer belt.
However, the transfer nip area as formed above causes various problems. Since the surface of the first transfer roller is more flexible than that of the photoreceptor drum, potential fluctuations in traveling speed of the intermediate transfer belt in the transfer nip area lead to an imbalance in peripheral speed between the intermediate transfer belt and the photoreceptor drum, thereby causing difficulty in proper transfer of a toner image. The fluctuations in traveling speed are likely to be caused by changes, with time or due to environmental changes, in coefficient of friction between the intermediate transfer belt and the photoreceptor drum.
Also, width of the transfer nip area needs to be increased in order to ensure that a toner image is transferred from the photoreceptor drum to the intermediate transfer belt. The intermediate transfer belt is thus pressed closely against the photoreceptor drum, so that part of toner particles are clumped together. When a toner image is transferred to a sheet, the clumped toner particles remain on the intermediate transfer belt, thereby causing a void, or absence of toner within a specified outline of a character or the like, in the transferred image on the sheet. This results in deterioration in image quality.
Besides, with the intermediate transfer belt pressed closely against the photoreceptor drum, toner residues originating upstream on the intermediate transfer belt are likely to be attracted to a photoreceptor drum positioned downstream. This results in undesirable mixture of toner of different colors, causing a discrepancy in color between an original image and an image as formed based thereon.
In view of the foregoing, Applicants have offered a transfer device as shown in FIG. 1. In the transfer device, first transfer rollers 13A to 13D are arranged in first transfer regions TA to TD, respectively, so as to be positioned downstream of respective transfer nip areas in a traveling direction of an intermediate transfer belt 11 as indicated by an arrow A. The first transfer rollers 13A to 13D are out of contact with photoreceptor drums 101A to 101D, respectively, through the intermediate transfer belt 11. The transfer nip areas are provided over a predetermined contact width in the traveling direction of the intermediate transfer belt 11 and the photoreceptor drums 101A to 101D, respectively. This arrangement prevents the fluctuations in traveling speed of the intermediate transfer belt 11 in the transfer nip areas, the deterioration in image quality caused by the clamped toner particles, and the mixture of toner of different colors. This arrangement also prevents wasteful consumption of toner.
In the transfer device as shown in FIG. 1, however, the transfer nip area in the first transfer region located most upstream on the intermediate transfer belt 11 in the traveling direction (or the most upstream first transfer region) has a width (or contact width in the traveling direction of the intermediate transfer belt 11 and the photoreceptor drum) narrower than those of the other transfer nip areas in the other first transfer regions.
A bottommost portion of a circumferential surface of a driven roller, which is arranged upstream of the most upstream first transfer region and over which the intermediate transfer belt 11 is stretched, is at a higher level than a bottommost portion of a circumferential surface of each of the first transfer rollers. Upstream of the photoreceptor drum in the most upstream first transfer region, therefore, the intermediate transfer belt 11 follows a path different from the one that the belt 11 follows upstream of the photoreceptor drums in the other three first transfer regions.
More specifically, the intermediate transfer belt 11 is approximately level in the first three transfer regions while the traveling path is slanted in the most upstream first transfer region.
The condition prevents the four first transfer regions from producing uniform transfer results, thereby causing a problem of deterioration in color image reproducibility.
In view of the foregoing, a feature of the present invention is to offer a transfer device having a constant contact width of an intermediate transfer belt and each of photoreceptor drums in each of a plurality of first transfer regions, or a constant transfer nip width. The construction of the device allows uniform transfer results to be achieved in the first transfer regions, thereby enhancing image reproducibility.